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The independent variable is the reaction-mixture temperature, not simply the bath setting. Bring separate reactant portions to the chosen temperature in a water bath before mixing, then record the mixture temperature. Keep concentrations, total volume, mixing, catalyst quantity and the chosen endpoint constant. A clock reaction gives a visible change after a small, approximately fixed amount of product has formed.
Video by Royal Society of Chemistry. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Control temperature in a rate investigation
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which quantity should be recorded as the reaction temperature?
The measured mixture temperature. Bath and mixture temperatures can differ, especially just after mixing.
After watching 2: When is 1/t a justified relative-rate proxy?
When the same small reaction extent defines each endpoint. A fixed threshold makes time inversely related to early reaction rate.
What you'll be able to do
The independent variable is the reaction-mixture temperature, not simply the bath setting. Bring separate reactant portions to the chosen temperature in a water bath before mixing, then record the mixture temperature. Keep concentrations, total volume, mixing, catalyst quantity and the chosen endpoint constant. A clock reaction gives a visible change after a small, approximately fixed amount of product has formed.
Use the teacher-approved clock mixture, small quantities and the specified waste container. Do not increase temperature beyond the approved range: heating can affect hazards, evaporation and side reactions as well as rate. A reaction that finishes too quickly for reproducible mixing and timing is a poor choice for manual measurement.
If every run reaches the same small extent of reaction, 1/t is proportional to an average early rate. It is a relative-rate proxy, with units s⁻¹, rather than automatically a rate in mol dm⁻³ s⁻¹ or a rate constant. Record individual times, repeat each temperature, and assess scatter before drawing a smooth trend. An actual amount threshold is needed to calculate a chemical production rate.
Temperature can drift after mixing. Measure it consistently and improve equilibration or bath control when drift is appreciable. If manual timing has roughly fixed absolute uncertainty, its fractional effect is larger for a 10-second run than a 100-second run. An objective colour sensor or data logger may improve endpoint detection and timing.
| Temperature / K | Clock time / s | Relative rate, 1/t / s⁻¹ |
|---|---|---|
| 293 | 80.0 | 0.0125 |
| 303 | 40.0 | 0.0250 |
| 313 | 22.0 | 0.0455 |
At higher temperature, particles have a higher mean kinetic energy and a greater fraction have energy at least the activation energy. This normally increases the successful-collision rate. It does not reduce activation energy for an unchanged reaction mechanism. A rate doubling for a 10 °C rise may occur over a particular interval, but is not a universal rule.
For an Arrhenius analysis use kelvin, plot ln k against 1/T and use the gradient −Ea/R. Replacing k by 1/t needs a justified proportionality with the same other factors across runs. Do not label such a graph an exact rate-constant determination without establishing that assumption.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Timing uncertainty is ±0.5 s. Compare its percentage contribution for 10.0 s and 100.0 s runs.
The short run has ten times the relative timing uncertainty.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.